The primary goal of this study is to explore the role of this purine nucleotide in children who have suffered severe head injury and exhibit altered cerebral blood flow (CBF) and metabolism. We believe that adenosine may contribute to the changes in CBF and CMRO2 of children after severe traumatic brain injury and since this level can be measured in the cerebrospinal fluid (CSF), we anticipate that concentrations of adenosine will be increased in CSF of children exhibiting hyperemic CBF compared to controls.